The present invention relates to a novel vascular allograft.
Since about the year 1900, much research effort has been undertaken to provide substitute vascular conduits in surgical situations. In essence, a substitute or artificial vein for the natural vein in a mammalian species provides oxygen to blood tissues and removes venous blood from the tissues in the same manner. Many situations have been identified where substitute vessels are needed:
1. Replacement vessels are needed for supplying ischemic tissue, which cannot obtain enough oxygen and nutrients to stay alive. Such a vessel may be required in cases of injury to a blood vessel which cannot be primarily repaired, or when a vessel is excised as a result of tumor mass or other obstructions.
2. A bypass vessel may be employed to circumvent an obstruction and may be emplaced by an end-to-side connection. Thus, blood is permitted to flow around the obstruction through this expedient.
3. Collateral vessels may be employed to provide circulation to ischemic tissue without disrupting inefficient vessels.
4. A shunt may be devised, such as one needed for renal dialysis. For example, a vessel may be placed in the forearm of a human in a subcutaneous position to permit venepuncture allowing dialysis to proceed.
5. A vessel extender may be provided in cases of free tissue transfer. In other words, such extenders may be employed where the existing pedicle of the composite graft is not sufficiently long.
In general, the mammalian circulation system operates when the heart pumps blood carrying oxygen and nutrients through arteries to all of the tissues of the body. Subsequently, oxygen-deprived blood and metabolic byproducts are returned by the veins to replenish oxygen for passing such metabolic byproducts to waste. The veins have lower blood pressure than the arteries. Thus, the blood returns to the heart by back pressure from the arterial system, muscular action in the limbs, and gravity in the case where portions of the body lie above the heart. Low venus blood from the lower limbs is facilitated by one-way valves in the walls of the veins. For example, the saphenous veins permit flow toward the heart but prevent backflow therefrom. Thus, veins in this aspect constitute a one-way flow system.
In the past, it has been found that autogenous grafting has achieved success without evidence of graft rejection. In allografts, however, tissue removed from one person and implanted in another person typically has resulted in graft rejection, requiring the use of immunosuppressive drugs. It should be noted that the use of heterografts (xenografts) has been uniformly unsuccessful due to rapid rate of graft rejection. Within the last 50 years, tissue banks supplying frozen tissue have been established. Such banks, however, have not completely solved the problem of antigenic rejection with respect to allograft tissue.
Modified vascular conduits have been provided utilizing chemical preservatives such as glutaraldehyde in conjunction with a mesh frame work. Reference is made to U.S. Pat. Nos. 3,988,782 and 3,974,526 in which a process for producing tubular prostheses is described. Unfortunately, such preservation system is relatively short-lived.
Artificial grafts, such as ones constructed of polytetrafluoroethylene (PTFE) have proved successful when used with larger diameter vessels, but unsuccessful in smaller diameter vessels. Further, anastomosis between the PTFE graft and different sized vessels is difficult. In addition, the PTFE grafts are rigid and don't propagate the pulse within the body. Moreover, PTFE grafts tend to kink if bent beyond a certain degree.
Autologous grafts are commonly used in heart bypass surgery. That is to say, saphenous veins from human legs have been harvested and transplanted into the heart as a bypass vessel. In addition, such autologous grafts have also been used as a dialysis shunt, for free tissue transfers, and as an added vascular pedicle. Although successful in many aspects, saphenous veins are usually thin walled and hard to handle. During harvesting procedures, saphenous vessels possess tributaries which must be tied off causing flow turbulence in the patients circulation system. Also, saphenous veins have varying widths and may include one-way valves which must be eliminated. In addition, extra operating time is required for harvesting vessels from a patient's leg prior to implantation. After harvesting, lower limb complications may occur in the patient such as delayed healing or painful scars in the leg. Moreover, arteries are not usually available for use as an autograft. Arteries harvested from a cadaver for use as an allograft have exhibited graft rejection problems. However, arteries if usable, are easier to handle because they are thicker walled, contain no one-way valves, and possess less tributaries to be tied off or isolated. It has been found historically that arteries, specifically the media layer, will spasm if used as a graft leading to occlusion of the vessel.
Lyophilization of human and other mammalian tissue has been employed to preserve bone, fascia, tendons, cartilage, ligaments, and the like. U.S. Pat. Nos. 5,656,498 and 5,690,963 described freeze-drying of blood cells and the like for reuse. Several publications entitled Rat Epigastric Pedicle Model: A Clinically Relevant Evaluation Of 1-mm PTFE Grafts, Barttelbort et al; Microsurgical Application Of Freeze-Dried Venous Allografts, Pratt et al; Microsurgical Application Of Freeze-Dried Arterial Allografts, Pratt et al; and Experimental Freeze-Dried Microarterial Allografts In Rabbits, Pratt et al indicate that lyophilization may reduce or eliminate graft rejection in femoral arteries in rats and rabbits.
U.S. Pat. No. 4,239,492 described a method of preparing vascular grafts from umbilical cords using chemical preservatives.
U.S. Pat. No. 4,801,299 describes the processing of umbilical cords which are treated with using a detergent and are lyophilized just prior to implantation.
U.S. Pat. No. 5,131,908 in which a mandrel is employed to control the size and shape of a vessel to be preserved. Multiple chemical entities are applied to the vessel for cleaning and denaturing without lyophilization in the processing stage. The preservation technique described in this patent is primarily designed to denature the vessel in order to remove all cellular components, leaving an extra cellular matrix for implantation.
The provision of a preserved vessel isolated from the human umbilical cord for use as an allograft would be a notable advance in the medical field.